Membrane module for gas transfer and membrane supported biofilm process

ABSTRACT

An apparatus to transfer gas to or from a liquid has a flexible and oxygen permeable but liquid water impermeable membrane, a flexible and gas permeable spacer, an inlet conduit, an outlet conduit and a non-rigid restraint system. When used for treating wastewater, an aerobic biofilm is cultured adjacent the planar elements, an anoxic biofilm is cultivated adjacent the aerobic biofilm and the wastewater is maintained in an anaerobic state. A first reactor for treating wastewater has an anaerobic section, a plurality of gas transfer membrane modules, and an aerobic section. A biofilm is cultivated on the surface of the gas transfer membranes in fluid communication with the anaerobic section. Biological reduction of COD, BOD, nitrogen and phosphorous are achieved. In a second reactor, phosphorous is also removed chemically in a precipitation branch.

This application claims the benefit of U.S. Provisional Application No.60/188,023, filed March 9, 2000, and U.S. Provisional Application No.60/189,498, filed March 15, 2000.

FIELD OF THE INVENTION

This invention relates to membrane modules used to transfer a gas to orfrom a liquid and to a process using a membrane supported biofilm fortreating wastewater to remove one or more of nitrogen, phosphorous, BODand COD.

BACKGROUND OF THE INVENTION

Transferring gases to or from a liquid is most commonly practiced byproviding a bubble diffuser in the liquid. As bubbles rise through theliquid, gases move across the boundary of the bubble driven by therelative partial pressures of the gas in the bubble and in the liquid.Such a process has serious drawbacks including high energy costs,difficulty in independently controlling mixing of the liquid, foaming onthe liquid surface and lack of control over the gas released by thebubbles as they break at the liquid surface. Gas permeable membranemodules provide an alternate means for transferring a gas to or from aliquid and have been used in various reactor designs. Some examples aredescribed below.

U.S. Pat. No. 4,181,604 (issued to Onishi et al. on Jan. 1, 1980),describes a module having several loops of hollow fibre membranesconnected at both ends to a pipe at the bottom of a tank containingwastewater. The pipe carries a gas containing oxygen to the lumens ofthe membranes. Oxygen flows through the membranes to the wastewater andto an aerobic biofilm growing on the outer surface of the membranes. InU.S. Pat. No. 4,746,435 (issued to Onishi et al. on May 24, 1988), thesame apparatus is used but the amount of oxygen containing gas iscontrolled to produce a biofilm having aerobic zones and anaerobiczones.

U.S. Pat. No. 4,416,993 (issued to McKeown on Nov. 22, 1983), describesa membrane module in the form of a hollow plate. The plates are made ofa rigid frame wrapped in a porous “netting” made of PTFE laminated to awoven nylon fabric. The plates are attached to an overlapping stripwhich has an inlet port and an outlet port.

In “Bubble-Free Aeration Using Membranes: Mass Transfer Analysis”(Journal of Membrane Science, 47 (1989) 91-106) and “Bubble-FreeAeration Using Membranes: Process Analysis” (Journal Water PollutionControl Federation, 1988, Volume 60, Number 11, 1986-1992), Côtéet al.describe the use of silicone rubber tubes to transfer oxygen to waterwithout creating bubbles in the water. The apparatus for these studiesincludes a module having vertically oriented tubes suspended between aninlet header and an outlet header. The module is immersed in a tankcontaining water recirculated by a pump to provide a horizontal currentin the tank.

U.S. Pat. No. 5,116,506 (issued to Williamson et al. on May 26, 1992)describes a reactor having a gas permeable membrane dividing the reactorinto a gas compartment and a liquid compartment. The gas compartment isprovided with oxygen and methane which diffuse through the membrane tosupport a biofilm layer in the liquid compartment. The membrane is madeof a teflon and nylon laminate commonly known as Gore-tex (TM). In oneembodiment, the membrane divides the reactor into lower and upperportions. In another embodiment, the gas compartment rotates within theliquid compartment.

In “Studies of a Membrane Aerated Bioreactor for Wastewater Treatment”(MBR 2-Jun. 2, 1999, Cranfield University), Semmens et al. describe amembrane module having microporous polypropylene hollow fibres stitchedtogether to form a fabric. The fabric is mounted between a gas inletheader and a gas outlet header such that the fibres are orientedhorizontally. The module is immersed in water in an open reactor withwater recirculated by a pump to provide a horizontal current in thereactor.

Despite the variety of designs available, gas transfer membranes havenot achieved widespread commercial success. Common criticisms of modulesor reactors include (a) that membrane materials lack sufficient strengthto be durable in hostile environments (b) that membrane surface area isinadequate, particularly for a tank of a fixed and pre-selected size,(c) that excessive movement of liquid is required which is costly toimplement in large systems, (d) that biofilm growth on the membranes isdifficult to prevent or maintain at a controlled thickness and (e) thateven small leaks or defects in the membranes cause a significant loss ofsystem capacity.

Gas transfer is used for a number of processes, one of which iswastewater treatment. Discharging wastewater containing large amounts ofcarbon (BOD or COD), nitrogen and phosphorous into a natural body ofwater causes eutrophication, algae blooms, pollution and healthproblems. Various processes have been developed to treat wastewater toremove some or all of the carbon, nitrogen and phosphorous, some ofwhich will be summarized below.

Activated Sludge With Chemical Phosphorous Removal

In a typical activated sludge process, wastewater flows in seriesthrough an anoxic reactor, an aerobic reactor and a clarifier. Effluentfrom the clarifier is released to the environment. Activated sludge fromthe bottom of the clarifier is partially recycled to the anoxic reactorand partially wasted. Significant removal of nitrogen requires asignificant rate of recycle to alternately nitrify and denitrify thewastewater.

Phosphorous is removed by dosing soluble metal salts, such as ferricchloride or aluminum sulphate, at one or more points in the process intothe aerobic reactor to precipitate phosphate metal salts. The wastewater, however, contains many different ions which create undesirableside reactions. As a result, and particularly where very low effluenttotal phosphorus levels are required, precipitating phosphorous mayrequire the addition of 2-6 times the stoichiometric amount of the metalsalt. Accordingly, these processes result in high chemical costs, highsludge production, and a high level of metallic impurities in thesludge.

Activated Sludge with Biological Phosphorous Removal

Activated sludge techniques can also be modified to use microorganismsto store the phosphates. For example, U.S. Pat. No. 4,867,883 discussesa process which attempts to encourage the selection and growth of Bio-Porganisms which uptake phosphorus in excess of the amount normallyneeded for cell growth. Generally, the process consists of an anaerobiczone, an anoxic zone, an aerobic zone, and a clarifier. In the anaerobiczone, soluble BOD is assimilated and stored by the Bio-P organisms andphosphorus is released. Subsequently, in the anoxic and aerobic zones,the stored BOD is depleted and soluble phosphorous is taken-up in excessand stored as polyphosphates by the Bio-P organisms. In the clarifier,sludge containing phosphates settles out of the effluent. There is adenitrified recycle from the anoxic zone to the anaerobic zone, anitrified recycle from the aerobic zone to the anoxic zone, and anactivated sludge recycle from the clarifier to the anoxic zone. Thesludge recycle is done in multiple phases to ensure that nitrites arenot recycled to the anaerobic zone, which would limit phosphorousrelease. The biological mechanism by which bacteria release phosphorousin the anaerobic section involves the uptake of easily assimilatedorganic compounds such as volatile fatty acids (VFA). Depending on thelevel of VFA in the raw wastewater, an extra anaerobic section may beadded at the head of the process.

One problem with this process is that the settling characteristics ofthe sludge in the clarifier impose significant design limitations. Forexample, the process cannot operate at very high process solids levelsor high sludge retention times, particularly when high removal rates ofboth nitrogen and phosphorous are required. As a result, the system isgenerally considered to be inefficient and there is a high generationrate of waste sludge. In some cases, sand filters are added to the tailof the process to help remove solids carryover from an overloadedclarifier and reduce the amount of phosphorous in the effluent.

Another problem with this process is that there is a buildup ofphosphates in the system. The waste activated sludge contains Bio-Porganisms rich in phosphorous. When the organisms in the waste activatedsludge are digested, they release phosphorus which is typically returnedback to the process in the form of digester supernatant. Consequently,this reduces the efficiency of phosphorus removal in the process andresults in higher levels of phosphorus in the effluent. A partialsolution to this problem is to employ a side stream process called‘Phos-Pho Strip’ as described in U.S. Pat. No. 3,654,147. In thisprocess, activated sludge passes from the clarifier to a phosphorusstripper. In the stripper, phosphorus is released into the filtratestream by either: creating anaerobic conditions; adjusting the pH; orextended aeration. The resulting phosphate-rich filtrate stream passesto a chemical precipitator. The phosphate-free effluent stream is addedto the main effluent stream, the waste stream from the precipitatorcontaining the phosphates is discarded, and the phosphate-depletedactivated sludge is returned to the main process.

Membrane Bioreactor With Chemical Precipitation

A membrane bioreactor can be combined with chemical precipitationtechniques. In a simple example, precipitating chemicals are added to anaerobic tank containing or connected to a membrane filter. As above,however, dosages of precipitating chemicals substantially in excess ofthe stoichiometric amount of phosphates are required to achieve lowlevels of phosphates in the effluent. This results in excessive sludgegeneration and the presence of metallic precipitates which increase therate of membrane fouling or force the operator to operate the system atan inefficient low sludge retention time.

Membrane Supported Biofilm

U.S. Pat. No. 4,181,604 describes a module having several loops ofhollow fibre membranes connected at both ends to a pipe at the bottom ofa tank containing wastewater. The pipe carries a gas containing oxygento the lumens of the membranes through which the gas is supplied to thewastewater and to an aerobic biofilm growing on the outer surface of themembranes. In U.S. Pat. No. 4,746,435, the same apparatus is used butthe amount of oxygen containing gas supplied is controlled to produce abiofilm having aerobic zones and anaerobic zones and 1 to 7 ppm ofoxygen in the waste water. This process provides simultaneousnitrification and denitrification without sludge recirculation but nophosphorous removal.

U.S. Pat. No. 5,116,506 describes a reactor having an oxygen containinggas permeable membrane separating a reactor into a liquid compartmentand a gas compartment. The liquid compartment contains wastewater. Thegas compartment is provided with oxygen which diffuses through themembrane to support a biofilm layer. The biofilm layer has two parts, anaerobic layer adjacent the membrane and an anaerobic layer adjacent thewastewater. This process also provides simultaneous nitrification anddenitrification but again no phosphorous removal.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a membrane modulefor transferring a gas to or from a liquid. Such modules can be used,for example, in supporting and providing oxygen to a biofilm, in waterdegassing, in humidification, in pervaporation and to clean air. Anobject of the present invention is to provide a process for treatingwastewater to produce an effluent with reduced concentrations of one ormore of nitrogen, phosphorous and carbon (BOD or COD). These objects aremet by the combination of features, steps or both found in the claims.The following summary may not describe all necessary features of theinvention which may reside in a sub-combination of the followingfeatures or in a combination with features described in other parts ofthis document.

In one aspect, the invention provides an apparatus for transferring agas to or from a liquid having a flexible and gas diffusive but liquidwater impermeable membrane and a flexible spacer open to gas flow. Thespacer and the membrane together form a planar element with the membraneenclosing an inner space containing the spacer. One or more conduits areprovided for transferring gas between the inner space and the atmosphereor another location outside of the water and the inner space. One ormore tensile members or weights non-rigidly restrain the planar elementin a selected position in a selected reactor. Gases that may betransferred include oxygen, nitrogen, volatile organic compounds,hydrogen, and water vapour.

In another aspect, the invention provides a module for transferring agas to or from a liquid having a plurality of the apparatus describedabove and a gas manifold. The second ends of the gas inlet conduits areconnected in fluid communication with the manifold to admit gas to theplanar elements. The manifold is mounted above the water surface of areactor while the planar elements are located below the water surface ofthe reactor. The reactor has a tank having a generally straight flowpath covering a substantial portion of the tank between an inlet and anoutlet. The planar elements are restrained in positions in the reactorin which they are generally parallel to the flow path. In a wastewatertreatment applications, the reactor has a source of agitation foragitating the planar elements to release accumulated biofilm from timeto time.

In another aspect, the invention is directed at a process fortransferring a gas to or from a liquid comprising the steps of (a)immersing one or more of the planar elements described above in theliquid and (b) supplying a gas to the planar elements at a pressurewhich does not create bubbles in the liquid, the gas leaving the planarelements by diffusion or by forced circulation using a pump. For someembodiments, the pressure of the gas is preferably also less than thepressure of the wastewater against the planar elements.

In another aspect, the invention provides a hybrid wastewater treatmentreactor combining a membrane supported biofilm and suspended growthbiomass. The reactor has a first section containing a plurality of gastransfer membrane modules connected to an oxygen source and a secondsection having an oxygen source operable to create aerobic conditions inthe second section. In the first section, the supply of oxygen to themembrane modules is controlled to cultivate a biofilm on the surface ofthe membranes having aerobic and anoxic zones and to facilitatecultivation of an anaerobic mixed liquor in the first section generally.In the second section, the diffusers and oxygen source facilitatecultivation of an aerobic mixed liquor. Wastewater enters the reactorthrough an inlet to the first section and flows through the reactor soas to be treated in the anaerobic section, in the aerobic section and bycontact with the biofilm before leaving the reactor through asolid/liquid separator downstream of the second section. A portion ofthe settled sludge at the bottom of the clarifier is recycled to thefirst section.

Biological digestion of BOD, COD, nitrogen and phosphorous are achievedas summarized below:

Rough removal of BOD or COD and nitrogen occur in the biofilm.

Polishing denitrification and sludge reduction occur in the anaerobicmixed liquor.

Volatile fatty acids (VFA) are assimilated and phosphorous is releasedin the anaerobic mixed liquor.

polishing COD and BOD removal, polishing nitrification and biologicalphosphorous uptake occur in the aerobic mixed liquor.

phosphorous is extracted as excess biomass by wasting a portion of thesludge settled in the clarifier.

In another aspect, the invention provides a modified reactor in whichphosphorous is also extracted as a chemical precipitate. The anaerobicmixed liquor is most often quiescent allowing partial sedimentation ofthe anaerobic mixed liquor which produces a phosphorous rich solutionnear its surface. Alternatively, a portion of the anaerobic mixed liquoris treated in a solid-liquid separation device to produce a phosphorousrich solution. The phosphorous rich solution is treated in aprecipitation branch having a source of phosphorous precipitating agentssuch as metal salts and a precipitate separation device such as aclarifier or hydrocyclone.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be describedwith reference to the following figures.

FIGS. 1 and 2 show a first apparatus in elevation and sectional viewsrespectively.

FIGS. 3, 4 and 5 show a second apparatus in elevation, sectional andfront removed views respectively.

FIGS. 6 and 7 show a third apparatus in elevation and sectional viewsrespectively.

FIGS. 8 and 9 are schematic elevational representations of two reactorsfor use with the first, second or third apparatus.

FIGS. 10 and 11 are drawings of alternative configurations of the firstapparatus.

FIG. 12 is a schematic representation of a reactor for treatingwastewater.

FIG. 13 is a schematic representation of a second reactor for treatingwastewater.

DETAILED DESCRIPTION OF EMBODIMENTS

A First Embodiment

FIGS. 1 and 2 show a first apparatus 10 having a membrane 12, a spacer14, an inlet conduit 16, an outlet conduit 18, and a non-rigid restraintsystem 20.

The membrane 12 is a sheet material that can be sewed or glued into avariety of constructions. In the embodiment illustrated, a piece of thesheet material of an appropriate size, which may be made of severalsmaller pieces, is folded in half around the spacer 14 and fastened toitself with a line of stitching 22 or glue. All lines of stitching 22 ofthe first apparatus 10 (and all subsequent apparatuses described below)expected to be in contact with water are sealed by coating them withliquid silicone rubber or another waterproof adhesive. The membrane 12thus encloses an inner space 24 containing the spacer 14. The spacer 14and the membrane 12 together form a planar element 26.

The membrane 12 is flexible and gas diffusive but liquid waterimpermeable. By liquid water impermeable, we mean that a water moleculemay diffuse through the membrane 12 under a suitable driving force (forexample, if the gas within the inner space 24 is not at 100% humidity)but that water will not flow in the liquid state through the membrane12. A preferred membrane 12 is made of a woven or non-woven textilefabric, such as nylon, coated or impregnated with a gas permeable butwater impermeable layer. Silicone rubber is preferred for the layerbecause of its high permeability to oxygen and availability in liquidand spray forms but the layer must be inspected carefully to ensure thatit is free of voids. Alternative membranes may be constructed ofmicroporous hydrophobic materials which do not wet under typicalhydrostatic pressures such as polypropylene or PTFE. The spacer 14 isflexible and open to gas flow generally parallel to the membrane 12.Suitable materials are sold for use as spacers in reverse osmosismodules. For example, VEXAR (TM), a polypropylene expandable diamondmesh made by Valtex may be used.

The inlet conduit 16 and the outlet conduit 18 have first ends 16 a and18 a in fluid communication with the inner space 24. The inlet conduit16 and the outlet conduit 18 each also have second ends 16 b and 18 bextending outwardly from the first planar element 26. Waterproof glue isapplied to the point where the inlet conduit 16 and the outlet conduit18 exit from the planar element 26 to prevent water from leaking intothe inner space 24.

The inlet conduit 16 and the outlet conduit 18 are made of a compositeconstruction. A part near the second ends 16 b and 18 b of the conduits16 and 18 is a flexible solid tube. The second end 16 b of the inletconduit 16 has a releasable water tight connector to a header (notillustrated). The second end 18 b of the outlet conduit 18 may beexhausted to the atmosphere in some applications but may also becollected in a header (not illustrated). Each flexible tube ends shortlybelow the start of the spacer 14. From this point, each of the conduits16, 18 is made of a section of the spacer 14 or membrane 12. Asillustrated, the conduits 16, 18 are a section of the spacer 14 rolledto create a porous conduit which admits the flexible tube and extendsalong a side of the first planar element 26. Alternatively, the spacer14 may be folded over itself to form the conduits 16, 18 or a flexiblespring can be inserted into a tubular section of the membrane 12adjacent the spacer 14 to form conduits 16, 18.

Preferably, the inlet conduit 16 and outlet conduit 18 are located atopposed sides of the planar element 26 so that oxygen containing gasentering the inlet conduit 16 will travel across the planar element 26before leaving through the outlet conduit 18. Further preferably, eachof the conduits 16, 18 extends substantially along their respectiveopposed sides of the planar element 26 and are porous along asubstantial portion of their length inside of the planar element 26. Inthis way, the gas is encouraged to flow across the planar element 26 ina well distributed flow pattern. Optionally, gas can be encouraged toflow downwardly or, preferably, upwardly by placing the conduits 16, 18across the horizontal sides of the planar element 26 rather than thevertical sides of the planar element 26.

A drain tube 28 may also be provided having a first end in fluidcommunication with the bottom of the planar element 26 and a second endextending out of the planar element 26. The drain tube 28 is sealed withglue where it exits the planar element 26. The second end of the draintube 28 is provided with a fitting so that it can be connected to a pumpfor withdrawing water from the inner space 24 of the planar element 26.Under ideal conditions, such a drain tube 28 is not required. From timeto time, however, minute defects may develop in the planar element 26that admit small amounts of water. Further, under some conditions watervapour may condense and accumulate in the inner space 24. In eithercase, the use of a drain tube 28 avoids the need to periodically removethe first apparatus 10 to remove water from the inner space 24.Alternatively, the drain tube 28 can be inserted into the bottom of theplanar element 26 through the outlet conduit 18.

The restraint system 20 consists of a series of tensile members in theform of loops 30, preferably made of the same material as the membrane12 or another suitable fabric. The loops 30 are sewed or glued to theedges of the planar element 26 to provide a series of points ofattachment. Grommets, hooks or other fasteners might also be usedprovided that they distribute any expected load enough to avoid tearingthe edges of the planar element 26. The restraint system 20 permits theplanar element 26 to be fixedly but non-rigidly restrained in a selectedposition in a selected reactor by passing a wire or rope fixed to thereactor through the loops 30. In some cases, the wire or rope may assumea curved shape. In these cases, the lengths of the loops 30 arepreferably varied to accommodate the curved shape and so to transfer thetensile force to the planar element 26 evenly across the loops 30.Alternately, a larger number of tensioned wires or rope can be fitted atone end to a reactor and at the other end to the planar element 26 withclamping connectors such as those used to secure tarps. In this case,the edge of the planar element serves the purpose of the tensile memberand is reinforced as required.

An alternative version of the first apparatus 10′ is shown in FIG. 10.In this alternate version, a planar element 26′ is formed of a spacer14′ and a membrane 12′ assembled using a line of stitching 22′. Arestraint system 20′ has floats 32 sized to keep the top of the firstapparatus 10′ above a water surface. The bottom of the first apparatus10′ is kept submerged with tensile elements made of wires 34 a attachedto grommets 36. When the water is lowered or drained for maintenanceetc., second wires 34 b attached to grommets 36 perform the function ofthe floats 32 in restraining the top of the first apparatus 10′. Theinlet conduit 16′ is a short section at the top of the first apparatus10′ in which the spacer 14′ is exposed to the atmosphere. The outletconduit 18′ extends down one side and across the bottom of the firstapparatus 10′ but is only porous along the bottom of the first apparatus10′. The outlet conduit 18′ is attached to a suction pump to draw air inthrough the apparatus 10′ from top to bottom. Small amounts of waterentering the first apparatus 10′ are withdrawn periodically byincreasing suction to the outlet conduit 18′.

A plan view of another alternative version of the first apparatus 10″ isshown in FIG. 11. In this version, one or more planar elements 26″ ofthe first apparatus 10″ are wound in a spiral. The layers of the spiralare separated by one or more loose springs 38 or other open spacers,preferably spaced apart at regular intervals along the axis of thespiral. Gas enters and exists through conduits 16″ and 18″ but the orderthe relative locations of the conduits 16″ and 18″ illustrated may bereversed. The first apparatus 10″ is preferably mounted in a cylindricalvessel 39 which may be a tank or a large pipe. Flow of water through thevessel 39 may be made to follow the spiral of the first apparatus 10″ byplacing one of an inlet and outlet in the centre of the vessel and theother of the inlet and outlet at the perimeter of the vessel 39.Alternatively, flow of water through the vessel 39 may be made to beparallel to the axis of the spiral, for example where the vessel 39 is apipe, by providing an inlet at one end of the pipe, an outlet at anotherend of the pipe and placing the first apparatus 10″ in between the inletand outlet. Depending on the how tightly the first apparatus 10″ ispacked in the pipe, tensile members may not be required to restrain thefirst apparatus 10″ in position, but tensile members or anotherrestraint system are typically required where the vessel 39 is a largetank.

A Second Embodiment

FIGS. 3, 4 and 5 show a second apparatus 110 for supporting andoxygenating an immersed biofilm. The second apparatus 110 has a membrane112, a spacer 114, an inlet conduit 116, an outlet conduit 118, and anon-rigid restraint system 120.

The membrane 112 and spacer 114 are of the same material described forthe first embodiment. The membrane 112 is similarly folded around thespacer 114 and fastened to itself with a line of stitching 122 or glue.Additional lines of stitching 122 are used to fix the inlet conduit 116,outlet conduit 118 and second restraint system 120 in the positionsshown. The membrane 112 thus encloses an inner space 124 containing thespacer 114 and the spacer 114 and the membrane 112 together form aplanar element 126.

The inlet conduit 116 and the outlet conduit 118 have first ends 116 aand 118 a in fluid communication with the inner space 124. The inletconduit 116 and the outlet conduit 118 each also have second ends 116 band 118 b extending outwardly from the planar element 126. Waterproofglue is applied to the point where the conduits 116, 118 exit from thesecond planar element 126 to prevent water from leaking into the innerspace 124.

The inlet conduit 116 and the outlet conduit 118 are made of flexiblesolid tubes. The second end 116 b of the inlet conduit 116 has areleasable water tight connector to a header (not illustrated). Thesecond end 118 b of the outlet conduit 118 may be exhausted to theatmosphere in some applications but may also be collected in a header(not illustrated). Starting shortly below the start of the spacer 114each conduit has a plurality of perforations 40 to create a porousconduit. As for the first embodiment, the inlet conduit 116 and theoutlet conduit 118 are preferably located at opposed sides of the planarelement 126, extend substantially along their respective opposed sidesand are porous along a substantial portion of their length inside of thesecond planar element 126. Optionally, gas can be encouraged to flowdownwardly or, preferably, upwardly by placing the conduits 116, 118across the horizontal sides of the second planar element 126 rather thanthe vertical sides of the first planar element 126. A drain tube (notillustrated) may also be provided.

The restraint system 120 consists of a tensile member in the form of awire or rope 42 sewn or glued around a substantial part of the peripheryof the planar element 126. The wire or rope 42 sticks out of the planarelement 126 at a plurality of locations to provide points of attachment44. Preferably, four points of attachment 44 are provided, one in eachcorner of the planar element 126. The restraint system 120 permits theplanar element 126 to be fixedly but non-rigidly restrained in aselected position in a selected reactor by connecting the points ofattachment 44 to a reactor with ropes or wire. This attachment mayencourage the wire or rope 42 to assume a curved shape. In these cases,the relevant edges of the planar element 126 are made in a similarcurved shape.

A Third Embodiment

FIGS. 6 and 7 show a third apparatus 210. The third apparatus 210 has amembrane 212, a spacer 214, an inlet conduit 216, an outlet conduit 218,and a non-rigid restraint system 220.

The membrane 212 is a sheet material as described for the previousembodiments. The structure of the third apparatus differs, however, inthat the membrane 212 is folded around two layers of spacer 214separated by a flexible but impermeable separator 50, preferably aplastic sheet. The edges of the membrane are fastened together bywaterproof glue or a line of stitching 222 made waterproof with siliconerubber spray or glue. The membrane 212 thus encloses an inner space 224containing the spacer 214 and the spacer 214 and the membrane 212together form a planar element 226.

The inlet conduit 216 and the outlet conduit 218 have first ends 216 a,218 a in fluid communication with the inner space 224. The inlet conduit216 and the outlet conduit 218 also have second ends 216 b, 218 bextending outwardly from the planar element 226. In the third apparatus210, the conduits 216, 218 include a part of the planar element 226 anda header 52. The planar element 226 is potted in the header 52 with gasimpermeable glue 54 to make an airtight seal with the membrane 212 butleaving the spacer 214 in fluid communication with an inlet chamber 56and an outlet chamber 58 of the header 52. The inlet chamber 56 andoutlet chamber 58 are separated by the impermeable layer 50. The header52 provides an upper mount for fixedly attaching the top of the planarelement 226 in a selected position in a selected reactor.

Gas enters the third apparatus 210 through a tube 62 having one end influid communication with a gas source and a second end in fluidcommunication with the inlet chamber 56 of the header 52. From the inletchamber 56, the gas enters the planar element 226 through the exposededge of the spacer 214. The gas travels first downwards and then upwardsthrough the spacer 214. The gas exits the planar element 226 through theother exposed edge of the spacer 214 into the outlet chamber 58 of theheader 52 from which it leaves through several discharge ports 64 oralternately through a pipe to an outlet header (not illustrated). Adrain tube (not illustrated) may also be provided having a first end influid communication with the bottom of the planar element 226 and asecond end extending out of the planar element 226.

As the header 52 is intended to be mounted above water, a portion of themembrane 212 is either out of the water or in a depth of water that isnot sufficient to keep the membrane 212 pressed against the spacer 214.In this portion, preferably less than one half of the area of the planarelement 226, glues lines 66 substantially parallel to the primarydirection of gas flow attach the membrane 212 to the spacer at selectedintervals to prevent ballooning of the membrane 212. Similar glue linesmay be used in appropriate orientations if required in the firstapparatus 10 and second apparatus 110. In those cases, however, it ispreferred if the first apparatus 10 and second apparatus 110 aresubmerged deep enough in relation to the pressure of gas to be used toallow the water pressure to keep the membrane 212 against the spacer214.

The portion of the membrane 212 that is out of the water may permit somegas to diffuse to the atmosphere. Where the gas flowing within themembrane 212 is air, particularly air at a pressure below 10 kPa, thelength of membrane 212 that is out of the water can be controlled to thepoint where diffusion to the atmosphere is acceptable. Where a pure gassuch as oxygen flows within the membrane 212, however, diffusion to theatmosphere may be significant and the atmosphere exposed portion of themembrane 212 is preferably sealed with a gas impermeable coating.

The restraint system 220 consists of the header 52, which may be fixedlymounted in a reactor, and a weight 68 attached to the bottom of theplanar element 226. For this purpose, the membrane 212 extends below thebottom of the spacer 214 and the weight 68 is attached in two halves tothe membrane 212 by rivets 70 or other fasteners. The weight is of asufficient size to keep the planar element 226 hanging verticallydownwards from the header 52. Alternately, loops can be provided at thebottom of the third planar element 226 to allow attachment to the bottomof the reactor with ropes or wires.

Membrane Supported Biofilm Reactors for Wastewater Treatment

FIG. 8 shows a reactor 80 having a tank 82, a feed inlet 84 to the tank82, an effluent outlet 86 from the tank 82, a flow path 88 between thefeed inlet 84 and effluent outlet 86 and a plurality of the thirdapparatus 210. The third apparatus 210 is shown as an example only andthe second apparatus 110 or first apparatus 10 may also be used withsuitable modifications to the reactor 80.

The planar elements 226 are sized to fit the tank 82 and fill asubstantial amount of its volume. The planar elements 226 have nopre-manufactured or rigid frame and thus are preferably custom made toprovide efficient use of the available space in the tank 82. Forexample, planar elements 226 may range from 0.5 m to 2 m wide and 2 to10 m deep. The planar elements 226 are preferably arranged in the tank82 in a number of rows, one such row being shown in FIG. 8. The planarelements 226 may range from 0.5 to 2 mm in thickness and adjacent rowsare placed in the tank 82 side by side at a distance of 5 to 15 mm toallow for biofilm growth and wastewater flow between adjacent planarelements 226.

The tank 82 is longer than it is deep and it is preferred to encourage agenerally horizontal flow path 88 with minimal mixing. This is done byleaving some space near the ends (ie. near the inlet 84 and outlet 86)of the tank 82 for vertical movement of water and leaving minimal freespace at the top, bottom and sides of the tank 82. A baffle 90 may alsobe placed upstream of the effluent outlet 86 to force the flow path 88to go under it. A sludge outlet 92 is provided to remove excess sludge.

The flow path 88 is generally straight over a substantial portion of thetank 82 between the feed inlet 84 and effluent outlet 86. Each thirdapparatus 210 is held in the tank 82 by its headers 52 attached to aframe 90 and by its weight 68. The headers 52, frame 90 and weights 68restrain each third apparatus 210 in positions in the reactor 80 wherebythe planar element 226 of each third apparatus 210 are generallyparallel to the flow path 88. Preferably, a plurality of planar elements226 are spaced in series along the flow path 88 so that the reactor 80will more nearly have plug flow characteristics. Wastewater to betreated may be partially recycled from the effluent outlet 86 to thefeed inlet 84. Such a recycle can increase the rate of gas transfer byincreasing the velocity of wastewater along the flow path 88, but it ispreferred if the recycle ratio is small so as to not provide more nearlymixed flow characteristics in the reactor 80.

Oxygen containing gas is provided to each third apparatus 210 throughits inlet conduit 216 connected to an inlet manifold 94 located abovethe water to be treated. With the inlet manifold 94 located above thewater, a leak in any third apparatus 210 will not admit water into themanifold nor any other third apparatus 210. Gas leaves each thirdapparatus 210 through its outlet conduit 218 which is connected to anexhaust manifold 95. Although it is not strictly necessary to collectthe gases leaving each third apparatus 210, it does provide someadvantages. For example, the gas in the exhaust manifold 95 may havebecome rich in volatile organic compounds which may create odour orhealth problems within a building containing the reactor 80. These gasesare preferably treated further or at least vented outside of thebuilding.

Preferably, the gas is provided at a pressure such that no bubbles areformed in the water to be treated and, more preferably, at a pressure ofless than 10 kPa. This pressure is exceeded by the pressure of the waterto be treated from one meter of depth and beyond. Preferably at leasthalf of the area of the third planar elements 226 is below that depth.The water pressure thus prevents at least one half of the surface of themembranes 12 from ballooning.

Oxygen diffuses through the membranes 12. The amount of oxygen sodiffused is preferably such that an aerobic biofilm is cultured adjacentthe planar elements 226, an anoxic biofilm is cultivated adjacent theaerobic biofilm and the wastewater to be treated is maintained in ananaerobic state. Such a biofilm provides for simultaneous nitrificationand denitrification. A source of agitation 96 is operated from time totime to agitate the planar elements 226 to release accumulated biofilm.A suitable source of agitation is a series of coarse bubble aerators 98which do not provide sufficient oxygen to the water to be treated tomake it nonanaerobic.

FIG. 9 shows a second reactor 180 having a tank 182, a feed inlet 184,an effluent outlet 186, a flow path 188 and a plurality of the firstapparatus 10. The first apparatus 10 is shown as an example only and thesecond apparatus 110 or third apparatus 210 may also be used withsuitable modifications to the second reactor 180.

Each first apparatus 10 is held by its loops 30 wrapped around wires 100or ropes attached to the tank 182. The loops 30 and wires 100 restraineach first apparatus 10 in a position in the second reactor 180 wherebythe planar element 26 of each first apparatus 10 is generally parallelto the flow path 188.

The first planar elements 26 are sized to fit the tank 182 and fill asubstantial amount of its volume. Like the third planar elements 226,the first planar elements 26 have no pre-manufactured or rigid frame andare preferably custom made to provide efficient use of the availablespace in the tank 182. The first planar elements 26 may range from 0.25to 1 mm in thickness and are placed side by side at a distance of 5 to15 mm to allow for biofilm growth and wastewater flow between adjacentfirst planar elements 26.

The tank 182 is deeper than it is long and it is preferred to encouragea straight and generally vertical flow path 188 over a substantialportion of the tank 182 with minimal mixing. This is done by leavingminimal space near the ends and sides of the tank 82 but a substantialamount of space near the top and bottom of the tank 82. Water to betreated may be partially recycled from the effluent outlet 186 to thefeed inlet 184 but it is preferred that the recycle rate be small.

Oxygen containing gas is provided to each first apparatus 10 through itsinlet conduit 16 connected to a manifold 94 located above the water tobe treated. With the inlet manifold 94 located above the water, a leakin any first apparatus 10 will not admit water into the manifold nor anyother first apparatus 210. The outlet conduits 18 are clipped in aconvenient place, for example to the inlet manifold 94, above thesurface of the water to be treated. Preferably, the gas is provided at apressure of less than 10 kPa and the planar elements 26 are located morethan 1 m deep in the tank 182. In this way, the gas pressure is exceededby the pressure of the water to be treated which prevents the membranes12 from ballooning. Glue lines (not shown), preferably not effectingmore than one half of the area of the planar elements 26, can be used toreinforce part of the planar elements 26 if they can not be mounted deepenough.

Alternatively, gas flow through the first element 10 is produced byapplying a suction, preferably of not more than 10 kPa less thanatmospheric pressure, to the outlet conduits 18. The inlet conduits 16are placed in fluid communication with the atmosphere. By this method,the rate of gas diffusion across the membrane 12 is slightly reduced,but no reinforcement of the membrane 12 (for example, by glue lines) isrequired regardless of the depth of the first element 10.

Oxygen diffuses through the membranes 12 preferably such that an aerobicbiofilm is cultured adjacent the planar elements 26, an anoxic biofilmis cultivated adjacent the aerobic biofilm and the wastewater to betreated is maintained in an anaerobic state. A second source ofagitation 196 is operated from time to time to agitate the first planarelements 26 to release accumulated biofilm. A suitable source ofagitation is a series of mechanical mixers 102.

Other Reactors

The apparatus described above may also be used in alternative processesor arrangements. For example, gas transfer into a liquid can be achievedin a dead end configuration, ie. without an outlet conduit. In thiscase, however, it is preferable to provide a small outlet bleed toreduce condensation in the open space and vent gases transferred fromthe liquid into the open space of the apparatus. To remove gases from aliquid, a dead end configuration may also be used wherein no inletconduit is provided. Use of the apparatus in some other applications isdescribed below.

a) Water Degassing and Pervaporation.

In water degassing, water containing dissolved gases such as nitrogen,oxygen or carbon dioxide flows into a tank. Planar elements as describedabove are immersed in the tank. A sweep gas flows through the planarelement or a vacuum is applied to the planar element (the inlet conduitis omitted). Gases in the liquid cross the membrane to the inner spaceof the planar element from where they are removed through the outletconduit. Water lean in dissolved gases leaves the tank. Such a processis useful, for example, in producing ultrapure water. Pervaporation isaccomplished with a similar reactor but the feed water contains volatileorganic compounds which diffuse to the inner space of the planarelements.

b) Humidification

In humidification, planar elements are immersed in a water bath. Dry airenters the planar elements. Water vapour crosses the membrane to theinner space of the planar element and humid air leaves the planarelements.

c) Air Cleaning

In air cleaning, planar elements are immersed in a water bath enrichedwith nutrients and a biofilm is cultured on the planar elements. Aircontaining volatile organic compounds flows into the planar elements andthe volatile organic compounds diffuse through the membranes of theplanar elements to the biofilm. Air lean in volatile organic compoundsexits the planar elements.

Hybrid Membrane Supported Biofilm Process With Biological PhosphorousRemoval

FIG. 12 shows a second reactor 410 for treating wastewater having asecond tank 412 divided into first and second biological reactionsections which will be referred to as a membrane supported biofilm (MSB)section 414 and an aerated section 416 respectively. The two sections414, 416 may be provided in a single second tank 412 or in multipletanks.

The MSB section 414 has one or more gas transfer membrane modules 418connected to an oxygen supply 420. The oxygen supply 420 is typically apump drawing air from the atmosphere or a source of oxygen or oxygenenriched air. The oxygen supply 420 supplies an oxygen containing gas tothe membrane modules 418 at a pressure which causes oxygen to flowthrough the membrane modules 418. Oxygen flows through the membranemodules 418 by diffusion without creating bubbles. Suitable designs forsuch membrane modules 418 are known in the art. Examples are describedin U.S. Pat. No. 5,116,506 and in the preceding description of theapparatus 10, second apparatus 110 and third apparatus 210. The membranemodules 418 occupy between 2% and 20% of the volume of the MSB section414. The remainder of the MSB section 414 is occupied by anaerobic mixedliquor 426 in an anaerobic part of the MSB section 414 in fluidcommunication with the outside of the membrane modules 418.

Screened wastewater 422 to be treated flows through an inlet 424 intothe MSB section 414 wherein it becomes part of the anaerobic mixedliquor 426. Nutrients in the anaerobic mixed liquor 426 in combinationwith oxygen flowing through the membrane modules 418 cultivates abiofilm on the surface of the membrane modules 418. The oxygen supply420 is controlled to provide sufficient oxygen to maintain an aerobiczone within the biofilm, preferably directly adjacent to the membranemodules 418. The oxygen supply is not sufficient, however, to create anentirely aerobic biofilm. Anoxic and possible anaerobic zones are alsopresent in the biofilm, preferably in layers—the anoxic zone in a layeradjacent to the aerobic layer and the anaerobic zone, if any, adjacentto the anoxic zone. The oxygen supply is also not sufficient tooxygenate the anaerobic mixed liquor 426 which is in an anaerobic stateat least in a region around the membrane modules 418. The anaerobicmixed liquor 426 is periodically agitated by operating a mechanicalmixer 460, pumping through local recirculation loops or coarse bubbleaeration (designed to not transfer significant amounts of oxygen to theanaerobic mixed liquor 426) to prevent complete settling of theanaerobic mixed liquor 426 and to control the thickness of the biofilmattached to the membrane modules 418.

Anaerobic mixed liquor 426 flows through a passage in a partition 427 tothe aerated section 416 which is primarily an aerobic section. Bubbles428 of oxygen containing gas are introduced into the aerated section 416by diffusers 430 driven by a second oxygen supply 432, typically an airblower. The bubbles 428 are preferably fine and transfer oxygen to theanaerobic mixed liquor 426 making it a generally aerobic mixed liquor434. Alternatively, other suitable aeration devices or oxygen sourcesoperable to create aerobic conditions may be used in the second section.

A portion of the aerobic mixed liquor 434 is recycled to the MSB section414 by a pump 436 in a second passage or recycle loop 438. Anoxicconditions are created in a localized zone in the MSB section 414 wherethe recycled aerobic mixed liquor 434 first mixes with the anaerobicmixed liquor 426. Another portion of the aerobic mixed liquor 434 flowsto a clarifier 440 (or another liquid-solid separation device such as amembrane filter) and is separated into treated effluent 442 and settledactivated sludge 444. Part of the sludge 444 is recycled to the MSBsection 414 by a second pump 446 in a second recycle loop 448. Anotherpart of the sludge 444 is discarded or treated further as wasteactivated sludge 445. The clarifier 440 and sludge second recycle loop448 may be sized smaller than a clarifier in conventional activatedsludge systems to account for the portion of the total biomass that isattached as a film to the membrane modules 418. Similarly, the recycleloop 438 may be sized smaller than the aerobic to anoxic recycle in aconventional activated sludge process for biological nutrient removalbecause significant amounts of nitrification and denitrification occurin the biofilm attached to the membrane modules 418.

The MSB section 414 is a complex reactor comprising a plurality ofreaction zones. An aerobic reaction zone or section (usually signalledby the presence of dissolved oxygen) exists in the biofilm layer on themembrane modules. Anoxic zones or sections (usually signalled by thepresence of NO₃ but absence of dissolved oxygen) exist in the biofilmlayer and in the anaerobic mixed liquor 426 where the recycled aerobicmixed liquor 434 enters the MSB section 414. An anaerobic zone orsection (usually signalled by the absence of NO₃ and dissolved oxygen)exists in the anaerobic mixed liquor 426 generally. This collection ofreaction zones allows the following processes to occur in the MSBsection 414:

Rough removal of BOD or COD occurs in the biofilm.

Rough removal of nitrogen occurs in the biofilm, by means of alternatenitrification and denitrification in the aerobic and anoxic sections ofthe biofilm.

Polishing denitrification occurs in the anaerobic mixed liquor 426.

Volatile fatty acids (VFA) are produced by fermentation in the anaerobicmixed liquor 426.

Phosphorous is released and VFA are assimilated by Bio-P organisms inthe anaerobic mixed liquor 426.

Sludge is reduced anaerobically in the anaerobic mixed liquor 426.

Partial sedimentation of the anaerobic mixed liquor 426 produces aphosphorous rich solution near the surface of the aerobic mixed liquor426.

The bubble-aerated section 416 is a simpler reactor, but still providesmultiple functions including polishing COD and BOD removal, polishingnitrification and biological phosphorous uptake. These processescomplement those occurring in the MSB section 414. For example, cyclingmixed liquor between anaerobic and aerobic states promotes sludgereduction through digestion. The uptaken phosphorous is removed with thewaste activated sludge 445. The effluent 442 leaving the clarifier 440thus has reduced levels of all of COD, BOD, nitrogen and phosphorous.

Hybrid Membrane Supported Biofilm Process with Chemical PhosphorousRemoval

FIG. 13 shows a third reactor 510 similar in structure and function tothe reactor 510. In the third reactor 510, however, a chemicalprecipitation branch 450 is provided which receives fluid from theanaerobic mixed liquor 426, preferably from the top of the MSB section414. The inlet to the chemical precipitation branch 450 is located awayfrom the inlet 424 and the outlet from the recycle loop 438 so that thechemical precipitation branch 450 receives liquid from a truly anaerobicportion of the anaerobic mixed liquor 426. Further, the anaerobic mixedliquor 426 is not agitated, except periodically to remove biofilm fromthe membrane modules 418, and thus the anaerobic mixed liquor 426partially settles. The liquid near the top of the MSB section 414 isthus reduced in suspended biomass as well as being rich in dissolvedphosphorous released by suspended organisms moving from an aerobicenvironment (in the aerated section 416) to an anaerobic environment.Alternatively, a solids lean liquid can be extracted from the MSBsection 414 through a clarifier, membrane or other solids liquidseparation device which, although requiring additional equipment, doesnot require settling in the MSB section 414 and so the mixer 460 may beoperated continuously. Solids rich liquid from such liquid separationdevices is returned to the third reactor 510, preferably to the aeratedsection 416.

The liquid near the top of the MSB section 414 flows into aprecipitation line 454, typically by gravity although a pump may also beused. Metal salts 456 are added to the precipitation line 454 to createeither an amorphous sludge or a crystalline material that is removed ina clarifier 458 or other precipitate separation process such as ahydrocyclone. Because of the reduced amount of suspended biomass in theliquid extracted from the MSB section 414, and the higher concentrationof phosphorous relative to conventional activated sludge systems withchemical phosphorous removal, phosphorous can be precipitated with morenearly stoichiometric doses of the metal salts. The resulting effluentmay be either discharged or recycled to the third reactor 510,preferably to the aerated section 416, and the resulting sludge orcrystalline material may be either discarded or processed further.

Removing phosphorous in the chemical precipitation branch 450 reducesthe concentration of phosphorous in the waste activated sludge 445. Thisreduces the risk that phosphorous will be release through sludgeprocessing and recycled to the third reactor 510. Having segregated alower volume chemical sludge, its phosphorous content can be dealt withmore easily.

Embodiments similar to those described above can be made in manyalternate configurations and operated according to many alternatemethods within the teachings of the invention, the scope of which isdefined in the following claims.

We claim:
 1. A process for treating wastewater in a bioreactorcomprising the steps of, (a) providing one or more planar elementshaving no rigid frame and comprising (i) a flexible and gas permeablespacer, and (ii) a flexible and oxygen permeable but liquid waterimpermeable membrane enclosing an inner space containing the spacer; (b)fixedly but non-rigidly restraining the one or more planar elementsbelow the surface of the wastewater in the reactor; (c) flowing anoxygen containing gas through the planar elements at a pressure thatdoes not create bubbles in the wastewater to be treated but permitsoxygen to leave the planar elements by diffusion.
 2. The process ofclaim 1 wherein the pressure of the gas is less than 10 kPa.
 3. Theprocess of claim 1 wherein the pressure of the gas is less than thelowest pressure of the wastewater against the one or more planarelements.
 4. The process of claim 1 wherein an aerobic biofilm iscultured adjacent the planar elements, an anoxic biofilm is cultivatedadjacent the aerobic biofilm and the wastewater to be treated ismaintained in an anaerobic state.
 5. The process of claim 1 furthercomprising the step of agitating the planar elements from time to timeto release accumulated biofilm.
 6. A process for treating wastewater ina bioreactor comprising the steps of, (a) providing one or more planarelements having no rigid frame and comprising (i) a flexible and gaspermeable spacer, and (ii) a flexible and oxygen permeable but liquidwater impermeable membrane enclosing an inner space containing thespacer; (b) restraining the one or more planar elements below thesurface of the wastewater in the reactor; (c) flowing an oxygencontaining gas through the planar elements at a pressure less thanatmospheric.
 7. The process of claim 6 wherein the pressure of the gasis not more than 10 kPa below atmospheric pressure.
 8. The process ofclaim 6 wherein an aerobic biofilm is cultured adjacent the planarelements, an anoxic biofilm is cultivated adjacent the aerobic biofilmand the wastewater to be treated is maintained in an anaerobic state. 9.The process of claim 8 further comprising the step of agitating theplanar elements from time to time to release accumulated biofilm.
 10. Aprocess for treating wastewater to reduce concentrations of one or moreof BOD, COD, nitrogen and phosphorous comprising the steps of, (a)treating the wastewater through anaerobic digestion; (b) contacting thewastewater while it is generally in an anaerobic state with a biofilmhaving aerobic and anoxic zones; and, (c) treating the wastewaterthrough aerobic digestion.
 11. The process of claim 10 furthercomprising the step of separating suspended solids from the wastewater.12. The process of claim 11 wherein a portion of the suspended solidsseparated from the wastewater are recycled to the reactor.
 13. Theprocess of claim 10 wherein the steps of treating the wastewater throughanaerobic digestion and contacting the wastewater while it is generallyin an anaerobic state with a biofilm having aerobic and anoxic zones areperformed simultaneously.
 14. The process of claim 10 further comprisingthe steps of (a) allowing the wastewater being treated by anaerobicdigestion to settle at least periodically; (b) withdrawing solids leanwastewater from the wastewater being treated by anaerobic digestion; (c)precipitating compounds of phosphorous from the solids lean wastewater.